Introduction

dracut – low-level tool for generating an initramfs image

Create an initramfs for the kernel with the version . If is omitted, then the version of the actual running kernel is used. If is omitted or empty, then the default location /boot/initramfs-.img is used.

racut creates an initial image used by the kernel for preloading the block device modules (such as IDE, SCSI or RAID) which are needed to access the root filesystem, mounting the root filesystem and booting nto the real system.

At boot time, the kernel unpacks that archive into RAM disk, mounts and uses it as initial root file system. All finding of the root device happens in this early userspace.

If you are dropped to an emergency shell, while booting your initramfs, the file /run/initramfs/rdsosreport.txt is created, which can be safed to a (to be mounted by hand) partition (usually /boot) or a USB stick. Additional debugging info can be produced by adding rd.debug to the kernel command line. /run/initramfs/rdsosreport.txt contains all logs and the output of some tools. It should be attached to any report about dracut problems.

Command Syntax

dracut [OPTION…] [ []]

To create a initramfs image, the most simple command is:

# dracut

This will generate a general purpose initramfs image, with all possible functionality resulting of the combination of the installed dracut modules and system tools. The image is /boot/initramfs-.img and contains the kernel modules of the currently active kernel with version .

If the initramfs image already exists, dracut will display an error message, and to overwrite the existing image, you have to use the –force option.

# dracut –force

If you want to specify another filename for the resulting image you would issue a command like:

# dracut foobar.img

To generate an image for a specific kernel version, the command would be:

# dracut foobar.img 2.6.40-1.rc5.f20

A shortcut to generate the image at the default location for a specific kernel version is:

# dracut –kver 2.6.40-1.rc5.f20

If you want to create lighter, smaller initramfs images, you may want to specify the –hostonly or -H option. Using this option, the resulting image will contain only those dracut modules, kernel modules and filesystems, which are needed to boot this specific machine. This has the drawback, that you can’t put the disk on another controller or machine, and that you can’t switch to another root filesystem, without recreating the initramfs image. The usage of the –hostonly option is only for experts and you will have to keep the broken pieces. At least keep a copy of a general purpose image (and corresponding kernel) as a fallback to rescue your system.

Inspecting the Contents

To see the contents of the image created by dracut, you can use the lsinitrd tool.

# lsinitrd | less

To display the contents of a file in the initramfs also use the lsinitrd tool:

# lsinitrd -f /etc/ld.so.conf
include ld.so.conf.d/*.conf

Adding dracut Modules

Some dracut modules are turned off by default and have to be activated manually. You can do this by adding the dracut modules to the configuration file /etc/dracut.conf or /etc/dracut.conf.d/myconf.conf. You can also add dracut modules on the command line by using the -a or –add option:

# dracut –add bootchart initramfs-bootchart.img

To see a list of available dracut modules, use the –list-modules option:

# dracut –list-modules

Omitting dracut Modules

Sometimes you don’t want a dracut module to be included for reasons of speed, size or functionality. To do this, either specify the omit_dracutmodules variable in the dracut.conf or /etc/dracut.conf.d/myconf.conf configuration file, or use the -o or –omit option on the command line:

# dracut -o “multipath lvm” no-multipath-lvm.img

Adding Kernel Modules

If you need a special kernel module in the initramfs, which is not automatically picked up by dracut, you have the use the –add-drivers option on the command line or the drivers vaiable in the /etc/dracut.conf or /etc/dracut.conf.d/myconf.conf configuration file

# dracut –add-drivers mymod initramfs-with-mymod.img

Boot parameters

An initramfs generated without the “hostonly” mode, does not contain any system configuration files (except for some special exceptions), so the configuration has to be done on the kernel command line. With this flexibility, you can easily boot from a changed root partition, without the need to recompile the initramfs image. So, you could completly change your root partition (move it inside a md raid with encryption and LVM on top), as long as you specify the correct filesystem LABEL or UUID on the kernel command line for your root device, dracut will find it and boot from it.

The kernel command line usually can be configured in /boot/grub/grub.conf, if grub is your bootloader and it also can be edited in the real boot process in the grub menu.

The kernel command line can also be provided by the dhcp server with the root-path option. See the section called “Network Boot”. To get a quick start for the suitable kernel command line on your system, use the –print-cmdline option:

Specifying the root Device

This is the only option dracut really needs to boot from your root partition. Because your root partition can live in various environments, there are a lot of formats for the root= option. The most basic one is root=:

root=/dev/sda2

Because device node names can change, dependent on the drive ordering, you are encouraged to use the filesystem identifier (UUID) or filesystem label (LABEL) to specify your root partition:

root=UUID=19e9dda3-5a38-484d-a9b0-fa6b067d0331

or

root=LABEL=myrootpartitionlabel

To see all UUIDs or LABELs on your system, do:

# ls -l /dev/disk/by-uuid

or

# ls -l /dev/disk/by-label

If your root partition is on the network see the section called “Network Boot”.

Keyboard Settings

If you have to input passwords for encrypted disk volumes, you might want to set the keyboard layout and specify a display font.

Setting these options can override the setting stored on your system, if you use a modern init system, like systemd.

Blacklisting Kernel Modules

Sometimes it is required to prevent the automatic kernel module loading of a specific kernel module. To do this, just add rd.blacklist=, with not containing the .ko suffix, to the kernel command line. For example:

rd.driver.blacklist=mptsas rd.driver.blacklist=nouveau

The option can be specified multiple times on the kernel command line.

Speeding up the Boot Process

If you want to speed up the boot process, you can specify as much information for dracut on the kernel command as possible. For example, you can tell dracut, that you root partition is not on a LVM volume or not on a raid partition, or that it lives inside a specific crypto LUKS encrypted volume. By default, dracut searches everywhere. A typical dracut kernel command line for a plain
primary or logical partition would contain:

rd.luks=0 rd.lvm=0 rd.md=0 rd.dm=0

This turns off every automatic assembly of LVM, MD raids, DM raids and crypto LUKS.

Of course, you could also omit the dracut modules in the initramfs creation process, but then you would lose the posibility to turn it on on demand.

Injecting custom Files

To add your own files to the initramfs image, you have several possibilities.

This will put the contents of the rd.live.overlay directory into the root of the initramfs image.

The –install option let you specify several files, which will get installed in the initramfs image at the same location, as they are present on initramfs creation time.

# dracut –install ‘strace fsck.ext3 ssh’ initramfs-dbg.img

This will create an initramfs with the strace, fsck.ext3 and ssh executables, together with the libraries needed to start those. The –install option can be specified multiple times.

Network Boot

If your root partition is on a network drive, you have to have the network dracut modules installed to create a network aware initramfs image.

On a Red Hat Enterprise Linux or Fedora system, this means, you have to install the dracut-network rpm package:

# yum install dracut-network

The resulting initramfs image can be served by a boot manager residing on your local hard drive or it can be served by a PXE/TFTP server.

If you specify ip=auto on the kernel command line, then dracut asks a dhcp server about the ip adress for the machine. The dhcp server can also serve an additional root-path, which will set the root device for dracut. With this mechanism, you have static configuration on your client machine and a centralized boot configuration on your TFTP/DHCP server. If you can’t pass a kernel command line, then you can inject /etc/cmdline.d/mycmdline.conf, with a method described in the section called “Injecting custom Files”.

Reducing the Image Size

To reduce the size of the initramfs, you should create it with by ommitting all dracut modules, which you know, you don’t need to boot the machine.

You can also specify the exact dracut and kernel modules to produce a very tiny initramfs image.

For example for a NFS image, you would do:

# dracut -m “nfs network base” initramfs-nfs-only.img

Then you would boot from this image with your target machine and reduce the size once more by creating it on the target machine with the –host-only option:

# dracut -m “nfs network base” –host-only initramfs-nfs-host-only.img

This will reduce the size of the initramfs image significantly.

TROUBLESHOOTING

If the boot process does not succeed, you have several options to debug the situation. Some of the basic operations are covered here.

Identifying your problem area

1. Remove ‘rhgb’ and ‘quiet’ from the kernel command line

2. Add ‘rd.shell’ to the kernel command line. This will present a shell should dracut be unable to locate your root device

3. Add ‘rd.shell rd.debug log_buf_len=1M’ to the kernel command line so that dracut shell commands are printed as they are executed

4. The file /run/initramfs/rdsosreport.txt is generated, which contains all the logs and the output of all significant tools,which are mentioned later.

If you want to save that output, simply mount /boot by hand or insert an USB stick and mount that. Then you can store the output for later inspection.

Network root device related problems

This section details information to include when experiencing problems on a system whose root device is located on a network attached volume (e.g. iSCSI, NFS or NBD). As well as the
information from the section called “All bug reports”, include the following information:

· Please include the output of

# /sbin/ifup
# ip addr show

Debugging dracut

Configure a serial console

Successfully debugging dracut will require some form of console logging during the system boot. This section documents configuring a serial console connection to record boot messages.

1. First, enable serial console output for both the kernel and the bootloader.

2. Open the file /etc/grub.conf for editing. Below the line’timeout=5′, add the following:

serial –unit=0 –speed=9600
terminal –timeout=5 serial console

3. Also in /etc/grub.conf, add the following boot arguemnts to the’kernel’ line:

console=tty0 console=ttyS0,9600

4. When finished, the /etc/grub.conf file should look similar to the example below.

This will give you a dracut shell after the system pivot’ed back in the initramfs.

Command line Options

–kver

set the kernel version. This enables to specify the kernel version, without specifying the location of the initramfs image. For example:

# dracut –kver 3.5.0-0.rc7.git1.2.fc18.x86_64

-f|–force

overwrite existing initramfs file.

-m|–modules

specify a space-separated list of dracut modules to call when building the initramfs. Modules are located in /usr/lib/dracut/modules.d. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –modules “module1 module2” …

-o|–omit

omit a space-separated list of dracut modules. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –omit “module1 module2” …

-a|–add

add a space-separated list of dracut modules to the default set of modules. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –add “module1 module2” …

–force-add

force to add a space-separated list of dracut modules to the default set of modules, when -H is specified. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –force-add “module1 module2” …

-d|–drivers

specify a space-separated list of kernel modules to exclusively include in the initramfs. The kernel modules have to be specified without the “.ko” suffix. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –drivers “kmodule1 kmodule2” …

–add-drivers

specify a space-separated list of kernel modules to add to the initramfs. The kernel modules have to be specified without the “.ko” suffix. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –add-drivers “kmodule1 kmodule2” …

–omit-drivers

specify a space-separated list of kernel modules not to add to the initramfs. The kernel modules have to be specified without the “.ko” suffix. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in
quotes. For example:

# dracut –omit-drivers “kmodule1 kmodule2” …

–filesystems

specify a space-separated list of kernel filesystem modules to exclusively include in the generic initramfs. This parameter can be specified multiple times.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –filesystems “filesystem1 filesystem2” …

-k|–kmoddir
specify the directory, where to look for kernel modules

–fwdir

[:…]++
specify additional directories, where to look for firmwares. This parameter can be specified multiple times.

–kernel-cmdline

specify default kernel command line parameters

–kernel-only
only install kernel drivers and firmware files

–no-kernel
do not install kernel drivers and firmware files

–early-microcode
Combine early microcode with ramdisk

–no-early-microcode
Do not combine early microcode with ramdisk

–print-cmdline
print the kernel command line for the current disk layout

–mdadmconf
include local /etc/mdadm.conf

–nomdadmconf
do not include local /etc/mdadm.conf

–lvmconf
include local /etc/lvm/lvm.conf

–nolvmconf
do not include local /etc/lvm/lvm.conf

–fscks [LIST]
add a space-separated list of fsck tools, in addition to dracut.conf’s specification; the installation is opportunistic (non-existing tools are ignored)

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –fscks “fsck.foo barfsck” …

–nofscks
inhibit installation of any fsck tools

–strip
strip binaries in the initramfs (default)

–nostrip
do not strip binaries in the initramfs

–prelink
prelink binaries in the initramfs (default)

–noprelink
do not prelink binaries in the initramfs

–hardlink
hardlink files in the initramfs (default)

–nohardlink
do not hardlink files in the initramfs

–prefix

prefix initramfs files with the specified directory

–noprefix
do not prefix initramfs files (default)

-h, –help
display help text and exit.

–debug
output debug information of the build process

-v, –verbose
increase verbosity level (default is info(4))

-q, –quiet
decrease verbosity level (default is info(4))

-c, –conf
specify configuration file to use.

Default: /etc/dracut.conf

–confdir
specify configuration directory to use.

Default: /etc/dracut.conf.d

–tmpdir
specify temporary directory to use.

Default: /var/tmp

–sshkey
ssh key file used with ssh-client module.

-l, –local
activates the local mode. dracut will use modules from the current working directory instead of the system-wide installed modules in /usr/lib/dracut/modules.d. This is useful when running dracut from
a git checkout.

-H, –hostonly
Host-Only mode: Install only what is needed for booting the local host instead of a generic host and generate host-specific configuration.

Warning
If chrooted to another root other than the real root device, use “–fstab” and provide a valid /etc/fstab.

-N, –no-hostonly
Disable Host-Only mode

–hostonly-cmdline

Store kernel command line arguments needed in the initramfs

–no-hostonly-cmdline

Do not store kernel command line arguments needed in the initramfs

–persistent-policy

Use to address disks and partitions. can be any directory name found in /dev/disk. E.g. “by-uuid”, “by-label”

–fstab
Use /etc/fstab instead of /proc/self/mountinfo.

–add-fstab
Add entries of to the initramfs /etc/fstab.

–mount ” [ []]]”
Mount on with in the initramfs. , and can be specified, see fstab manpage for the details. The default is “defaults”. The default is “0”. the default is “2”.

–add-device
Bring up in initramfs, should be the device name. This can be useful in hostonly mode for resume support when your swap is on LVM or an encrypted partition. [NB –device can be used for compatibility with earlier releases]

-i, –include
include the files in the SOURCE directory into the TARGET directory in the final initramfs. If SOURCE is a file, it will be installed to TARGET in the final initramfs. This parameter can be specified multiple times.

-I, –install
install the space separated list of files into the initramfs.

Note : If [LIST] has multiple arguments, then you have to put these in quotes. For example:

# dracut –install “/bin/foo /sbin/bar” …

–gzip
Compress the generated initramfs using gzip. This will be done by default, unless another compression option or –no-compress is passed. Equivalent to “–compress=gzip -9”

–bzip2
Compress the generated initramfs using bzip2.

Warning : Make sure your kernel has bzip2 decompression support compiled in, otherwise you will not be able to boot. Equivalent to “–compress=bzip2”

–lzma
Compress the generated initramfs using lzma.

Warning : Make sure your kernel has lzma decompression support compiled in, otherwise you will not be able to boot. Equivalent to “lzma –compress=lzma -9”

–xz
Compress the generated initramfs using xz.

Warning : Make sure your kernel has xz decompression support compiled in,otherwise you will not be able to boot. Equivalent to “lzma –compress=xz –check=crc32 –lzma2=dict=1MiB”

–lzo
Compress the generated initramfs using lzop.

Warning : Make sure your kernel has lzo decompression support compiled in, otherwise you will not be able to boot.

–lz4
Compress the generated initramfs using lz4.

Warning : Make sure your kernel has lz4 decompression support compiled in,otherwise you will not be able to boot.

–compress
Compress the generated initramfs using the passed compression program. If you pass it just the name of a compression program, it will call that program with known-working arguments. If you pass a quoted string with arguments, it will be called with exactly those arguments. Depending on what you pass, this may result in an initramfs that the kernel cannot decompress.

–no-compress
Do not compress the generated initramfs. This will override any other compression options.

–list-modules
List all available dracut modules.

-M, –show-modules
Print included module’s name to standard output during build.

–regenerate-all
Regenerate all initramfs images at the default location with the kernel versions found on the system. Additional parameters are passed through.

Command Line Examples:

To create a initramfs image, the most simple command is:

# dracut

This will generate a general purpose initramfs image, with all possible functionality resulting of the combination of the installed dracut modules and system tools. The image is /boot/initramfs-.img and contains the kernel modules of the currently active kernel with version .

If the initramfs image already exists, dracut will display an error message, and to overwrite the existing image, you have to use the –force option.

# dracut –force

If you want to specify another filename for the resulting image you would issue a command like:

# dracut foobar.img

To generate an image for a specific kernel version, the command would be:

# dracut foobar.img 2.6.40-1.rc5.f20

A shortcut to generate the image at the default location for a specific kernel version is:

# dracut –kver 2.6.40-1.rc5.f20

If you want to create lighter, smaller initramfs images, you may want to specify the –hostonly or -H option. Using this option, the resulting image will contain only those dracut modules, kernel modules and filesystems, which are needed to boot this specific machine. This has the drawback, that you can’t put the disk on another controller or machine, and that you can’t switch to another root filesystem, without recreating the initramfs image. The usage of the –hostonly option is only for experts and you will have to keep the broken pieces. At least keep a copy of a general purpose image (and corresponding kernel) as a fallback to rescue your system.

Inspecting the Contents

To see the contents of the image created by dracut, you can use the lsinitrd tool.

# lsinitrd | less

To display the contents of a file in the initramfs also use the lsinitrd tool:

# lsinitrd -f /etc/ld.so.conf
include ld.so.conf.d/*.conf

Adding dracut Modules

Some dracut modules are turned off by default and have to be activated maually. You can do this by adding the dracut modules to the configuration file /etc/dracut.conf or /etc/dracut.conf.d/myconf.conf.. You can also add dracut modules on the command line by using the -a or –add option:

# dracut –add bootchart initramfs-bootchart.img

To see a list of available dracut modules, use the –list-modules option:

# dracut –list-modules

Omitting dracut Modules

Sometimes you don’t want a dracut module to be included for reasons of speed, size or functionality. To do this, either specify the omit_dracutmodules variable in the dracut.conf or /etc/dracut.conf.d/myconf.conf configuration file, or use the -o or –omit option on the command line:

# dracut -o “multipath lvm” no-multipath-lvm.img

Adding Kernel Modules

If you need a special kernel module in the initramfs, which is not automatically picked up by dracut, you have the use the –add-drivers option on the command line or the drivers vaiable in the
/etc/dracut.conf or /etc/dracut.conf.d/myconf.conf configuration file :

# dracut –add-drivers mymod initramfs-with-mymod.img

Boot parameters

An initramfs generated without the “hostonly” mode, does not contain any system configuration files (except for some special exceptions), so the configuration has to be done on the kernel command line. With this flexibility, you can easily boot from a changed root partition, without the need to recompile the initramfs image. So, you could completly change your root partition (move it inside a md raid with encryption and LVM on top), as long as you specify the correct filesystem LABEL or UUID on the kernel command line for your root device, dracut will find it and boot from it.

The kernel command line usually can be configured in /boot/grub/grub.conf, if grub is your bootloader and it also can be edited in the real boot process in the grub menu.

The kernel command line can also be provided by the dhcp server with the root-path option. See the section called “Network Boot”.

To get a quick start for the suitable kernel command line on your system, use the –print-cmdline option:

Specifying the root Device

This is the only option dracut really needs to boot from your root partition. Because your root partition can live in various environments, there are a lot of formats for the root= option. The
most basic one is root=:

root=/dev/sda2

Because device node names can change, dependent on the drive ordering, you are encouraged to use the filesystem identifier (UUID) or filesystem label (LABEL) to specify your root partition:

root=UUID=19e9dda3-5a38-484d-a9b0-fa6b067d0331

or

root=LABEL=myrootpartitionlabel

To see all UUIDs or LABELs on your system, do:

# ls -l /dev/disk/by-uuid

or

# ls -l /dev/disk/by-label

If your root partition is on the network see the section called “Network Boot”.

Keyboard Settings

If you have to input passwords for encrypted disk volumes, you might want to set the keyboard layout and specify a display font. A typical german kernel command would contain:

Setting these options can override the setting stored on your system, if you use a modern init system, like systemd.

Blacklisting Kernel Modules

Sometimes it is required to prevent the automatic kernel module loading of a specific kernel module. To do this, just add rd.blacklist=, with not
containing the .ko suffix, to the kernel command line. For example:

rd.driver.blacklist=mptsas rd.driver.blacklist=nouveau

The option can be specified multiple times on the kernel command line.

Speeding up the Boot Process

If you want to speed up the boot process, you can specify as much information for dracut on the kernel command as possible. For example, you can tell dracut, that you root partition is not on a LVM volume or not on a raid partition, or that it lives inside a specific crypto LUKS encrypted volume. By default, dracut searches everywhere. A typical dracut kernel command line for a plain primary or logical partition would contain:

rd.luks=0 rd.lvm=0 rd.md=0 rd.dm=0

This turns off every automatic assembly of LVM, MD raids, DM raids and crypto LUKS.

Of course, you could also omit the dracut modules in the initramfs creation process, but then you would lose the posibility to turn it on on demand.

Injecting custom Files

To add your own files to the initramfs image, you have several possibilities. The –include option let you specify a source path and a target path. For example

This will put the contents of the rd.live.overlay directory into the root of the initramfs image.

The –install option let you specify several files, which will get installed in the initramfs image at the same location, as they are present on initramfs creation time.

# dracut –install ‘strace fsck.ext3 ssh’ initramfs-dbg.img

This will create an initramfs with the strace, fsck.ext3 and ssh executables, together with the libraries needed to start those. The –install option can be specified multiple times.

Network Boot

If your root partition is on a network drive, you have to have the network dracut modules installed to create a network aware initramfs image.

On a Red Hat Enterprise Linux or Fedora system, this means, you have to install the dracut-network rpm package:

# yum install dracut-network

The resulting initramfs image can be served by a boot manager residing on your local hard drive or it can be served by a PXE/TFTP server.

If you specify ip=auto on the kernel command line, then dracut asks a dhcp server about the ip adress for the machine. The dhcp server can also serve an additional root-path, which will set the root device for dracut. With this mechanism, you have static configuration on your client machine and a centralized boot configuration on your TFTP/DHCP server. If you can’t pass a kernel command line, then you can inject /etc/cmdline.d/mycmdline.conf, with a method described in the section called “Injecting custom Files”.

Reducing the Image Size

To reduce the size of the initramfs, you should create it with by ommitting all dracut modules, which you know, you don’t need to boot the machine.

You can also specify the exact dracut and kernel modules to produce a very tiny initramfs image.

For example for a NFS image, you would do:

# dracut -m “nfs network base” initramfs-nfs-only.img

Then you would boot from this image with your target machine and reduce the size once more by creating it on the target machine with the –host-only option:

# dracut -m “nfs network base” –host-only initramfs-nfs-host-only.img

This will reduce the size of the initramfs image significantly.

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